Apo Color

I am hoping someone can give me some feedback. I recently bought an APM 80/600 apo. When viewing the moon (10 day old)with the terminator in the middle of the field the lunar limb (about 2/3rd to the edge of the field)at 75x (8mm Radian) shows a yellow rim. The star test is excellent. I thought a triplet should give better color correction across the whole field. The color correction is on a par with a William optic ZS 80ED which I used to compare. Is this normal? Tim

The colour is most likely from atmospheric refraction as the Moon is traveling pretty low in the sky from northern latitudes these days. A TMB 800/600 lens is very well corrected for colour, so you should see none at 75X.

If you have a reflector handy, try comparing them at the same time. If it is the atmosphere, you'll see it in the reflector too (use same eyepiece type if possible).

Philip

Tim,
First thing to check is the EP. The Radians do have a bit of CA and it can show at 2/3 out. So, try something like an Ortho that has no CA and see if the color is gone.

It's almost always lateral color in the eyepiece, especially if it's yellow on the limb at low power. Simplest thing to do is to leave the Moon right where it is in relation to your telescope, and physically move the eyepiece over sideways so that the edge of the Moon is now in the center of the eyepiece. Color will be gone. This tells you that the eyepiece introduces the lateral color. (so you can't move the eyepiece over sideways 'cause the 1.25" hole is round and not oval? well just remove the adapter and hold the eyepiece in your hand).

By the way, folks, this is the single most common problem that just about every beginner experiences when looking at the Moon for the first time at low powers with a wide field eyepiece. The other common problem is that stars look oblong or have flares coming out of them at low power (mysteriously this disappears at higher powers). Anyone like to guess what causes that? (how about the objective is out of collimation with a low power eyepiece and somehow senses that you placed a high power ocular into the focuser and rights itself magically so that the high power view is correct).

Rolando

Tim Hrutkay said:
the lunar limb (about 2/3rd to the edge of the field)at 75x (8mm Radian) shows a yellow rim.

Its the Radian eyepiece. They are notorious for this effect.
They achieve the long eyerelief by using a very long focus
eyepiece (the rear group of lenses) with a very strong barlow
in the front group. The off-axis lateral color comes largely
from the strong barlow lens. Your test with the lunar limb is
very difficult for any eyepiece -- perhaps the most difficult test
for lateral color -- so it shows up strongly for the Radian. On
other targets like star fields it is much less noticeable.

Some notes from my notebook:

6mm Pentax SMC ortho, bright star 80% towards field edge:
Airy disk round and white, first Airy ring slight
red tint on side towards field center. No coma.

6mm Radian, scope not moved:
Airy disk very elongated, green on side away from field
center, red on side towards field center, shift
from red to green about 1/2 Airy disk diameter.
First Airy ring completely red. A little coma.

12mm Pentax SMC ortho + TV 2x Barlow, scope not moved:
Airy disk and first ring very white. Trace of coma.

18mm Pentax SMC + TV 3x Barlow, scope not moved:
Airy disk nearly white, slight color shift,
much less than Radian. First Airy ring violet tint
towards field center. A little coma.

IIRC the full moon was almost a perfect fit inside the FOV of a Nagler 22 with an AP 2x barlow (in a 130/6 APO refractor.) As I recall the entire circular rim of the full moon showed a yellow rim of color all the way around.